The present invention relates to a novel triphenylamine derivative which can be preferably used as a hole-transporting material, e.g., for organic electroluminescence device and an organic electroluminescence device comprising the same.
As an organic electroluminescence device comprising an organic layer mainly comprising an organic compound provided interposed between a pair of electrodes, i.e., cathode and anode there had been generally used one comprising a single organic layer. In recent years, however, various organic electroluminescence devices comprising a plurality of organic layers each independently having a function such as emission of light and transportation of carrier (e.g., hole, electron) (as disclosed in C. W. Tang and S. A. VanSlyke, xe2x80x9cAppl. Phys. Lett.xe2x80x9d, 51, 913 (1987), C. Adachi, T. Tsutsui and S. Saito, xe2x80x9cAppl. Phys. Lett.xe2x80x9d, 55, 1489 (1989), J. Kido, M. Kimura, and K. Nagai, xe2x80x9cSciencexe2x80x9d, Vol. 267, 1332 (1995)).
Such an organic electroluminescence device has the following advantages:
(1) It can emit light with a high luminance at a low voltage as compared with the conventional devices mainly comprising inorganic material;
(2) Since the formation of the various layers can be accomplished not only by vacuum evaporation method but also by solution coating method and any method can be selected taking into account the structure of each of the various layers, the degree of freedom of device design is enhanced, making it possible to enlarge the surface of device; and
(3) A multi-color system can be provided by designing the organic molecules.
Examples of the various layers constituting the organic layer comprising a plurality of layers include light-emitting layer, hole-transporting layer capable of transporting hole, and electron-transporting layer capable of electron. These layers are each formed by the foregoing organic compounds having excellent various properties or by dispersing those organic compounds in an appropriate polymer binder.
However, the conventional organic electroluminescence devices are disadvantageous in that they exhibit insufficient stability and durability mainly attributed to (1) deterioration of organic compound itself due to Joule""s heat developed when the device is energized or (2) deterioration in the carrier injection efficiency between various layers due to the reduction in smoothness of interface caused by the crystallization of organic compound by Joule""s heat thus developed, and hence exhibit a drastically reduced luminance during a repeated use.
The foregoing problem is remarkable particularly with a hole-transporting material having a low heat resistance constituting a hole-transporting layer among the organic compounds constituting the foregoing various layers. It is not too much to say that the heat resistance of organic electroluminescence devices is determined by the heat resistance of such a hole-transporting material.
Under these circumstances, extensive studies have recently been made on the molecular structure of such a hole-transporting material to improve the heat resistance thereof.
For example, Adachi et al. attempted to improve the heat resistance by polymerizing a triphenylamine derivative known as a hole-transporting material such as N,Nxe2x80x2-diphenyl-N,Nxe2x80x2-bis(3-methylphenyl)-1,1xe2x80x2-biphenyl-4,4xe2x80x2-diamine (hereinafter abbreviated as xe2x80x9cTPDxe2x80x9d, which represents a dimer of triphenylamine) represented by the following formula (3-1). 
As a result, it was reported that a triphenylamine trimer (hereinafter referred to as xe2x80x9cHTM1xe2x80x9d) represented by the following formula (4): 
exhibits a high heat resistance and an excellent hole-transporting capacity (C. Adachi, K. Nagai and N. Tamoto, xe2x80x9cAppl. Phys. Lett.xe2x80x9d, 66 (20), 2679 (1995)).
Further, Tokito et al. also attempted to improve the heat resistance by polymerizing a triphenylamine derivative in a similar manner as described above.
As a result, it was clarified that a triphenylamine tetramer (hereinafter referred to as xe2x80x9cTPTExe2x80x9d) represented by the following formula (5-1): 
exhibits a high heat resistance and an excellent hole-transporting capacity (S. Tokito, H. Tanaka, A. Okada and Y. Taga, xe2x80x9cAppl. Phys. Lett.xe2x80x9d, 69(7), 878 (1996); S. Tokito, H. Tanaka, K. Noda, A. Okada and Y. Taga, xe2x80x9cMacromol. Symp.xe2x80x9d, 125, 181-188 (1997); JP-A-10-25473 (The term xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d)).
Further, JP-A-7-126226 discloses that a benzidine compound obtained by normalizing the biphenyl ring in the center of the foregoing TPTE into various divalent groups containing the biphenyl ring and normalizing the kind of substituents on various rings and the substitution positions therefor exhibits an excellent stability at the time of light emission and storage, as compared with low molecular weight hole-transporting materials such as the foregoing TPD.
The inventors made studies on the foregoing problems. It was found that the foregoing various polymerized compounds or their peripheral compounds certainly exhibit a stabilized heat resistance and stability as compared with the conventional TPD but leave something to be desired in their effects. In particular, these compounds cannot satisfy the high heat resistance required for on vehicle display devices.
In other words, on vehicle display devices must operate without any trouble even when continuously driven at a temperature as high as 85xc2x0 C. and a humidity as high as 85% RH for 240 hours. Thus, the on vehicle display device, if it is in the form of organic electroluminescence device, must not show a drastic drop of luminance or stop light emission even when continuously operated under the foregoing high temperature and humidity conditions for 240 hours.
However, HTM1 (a trimer of triphenylamine) represented by the foregoing formula (4) exhibits a glass transition temperature Tg of about 110xc2x0 C. and hence shows a difference as small as 25xc2x0 C. from the foregoing ambient temperature, demonstrating that it exhibits an insufficient heat resistance. Thus, if continuously operated at such a high temperature and humidity, the foregoing generation of Joule""s heat causes the temperature of the device itself to exceed the glass transition temperature Tg of HTM1 in an extremely short period of time, resulting in deterioration of the foregoing organic compound itself, i.e., HTM1 itself, or reduction in the injection efficiency between layers.
Further, the thickness of the organic layer constituting the device is as extremely small as about 0.1 xcexcm in total, even if the organic layer comprises a plurality of layers. Thus, if there is some dispersion of thickness, current is concentrated into the section having the smallest thickness to cause local rise in temperature, resulting in the cracking and peeling of the organic layers that cause short-circuiting between the cathode and the anode.
It is thus expected that a device comprising HTM1 shows a drastic drop in luminance or stops light emission in a short period of time due to the foregoing defects.
The insufficient heat resistance of the organic electroluminescence device using HTM1 is also apparent from the fact described in the report by Tokito et al. that a device comprising TPTR, which is different from HTM1 only in the substitution position of terminal methyl group, has a critical temperature as described below of about 110xc2x0 C.
This is also applicable to TPTE represented by the foregoing formula (5-1). In other words, TPTE exhibits a glass transition temperature Tg of 130xc2x0 C. and hence shows a temperature difference as small as 45xc2x0 C. from the foregoing ambient temperature of 85xc2x0 C. Thus, if continuously operated at such a high temperature and humidity, it can be expected that the resulting deterioration of TPTE itself or the injection efficiency between layers or the short-circuiting between the cathode and the anode causes the device to show a drastic drop of luminance or to stop light emission in a short period of time.
The above described reference discloses that the upper limit of temperature at which an organic electroluminescence device comprising TPTE can operate is 140xc2x0 C. However, this temperature merely indicates the temperature (critical temperature) at which light emission stops when the ambient temperature is gradually raised while the device is being allowed to emit light. Thus, the temperature at which the device can stably and continuously emit light over an extended period of time was not confirmed therein.
According to the inventors"" study, the organic electroluminescence device comprising TPTE is disadvantageous in that it exhibits too small an external quantum efficiency to emit light with a high luminance at a small current.
This phenomenon is considered to be attributed to the fact that TPTE forms, for example in a multi-layer structure device, an exciplex with an electron-transporting material such as tris(8-quinolilato) aluminum (III) complex (hereinafter referred to as xe2x80x9cAlqxe2x80x9d) represented by the following formula (6): 
contained in the adjacent electron-transporting layer due to the interaction at the interface of the layers.
In an attempt to enhance the external quantum efficiency of devices by preventing the formation of such an exciplex, Noda et al. made a study on the molecular structure of TPTE. As a result, it was clarified that the foregoing object can be accomplished by employing a meta-position-linkage structure represented by the following formula (5-2): 
(Koji Noda, Hisayoshi Fujikawa, Katsunori Koda, Hisato Takeuchi, Seiji Tokito, Yasukuni Taga, xe2x80x9cPreprint of 45th Joint Forum of Society of Applied Physicsxe2x80x9d, Tokyo Engineering University, March 1998).
However, the foregoing meta-position-linkage TPTE (hereinafter referred to as xe2x80x9cm-TPTExe2x80x9d) is disadvantageous in that it exhibits a glass transition temperature as low as about 90xc2x0 C. and thus cannot be used for the purpose requiring a high heat resistance and reliability as in the foregoing on vehicle display device.
An object of the present invention is to provide a novel triphenylamine derivative which has further excellent heat resistance as compared to the conventional compounds and which is not accompanied with concern about easy formation of an exciplex with an electron-transporting material.
Another object of the present invention is to provide an organic electroluminescence device which comprises the triphenylamine derivative and thus has a high heat resistance and a high luminous efficiency at the same time.
Other objects and effects of the invention will become apparent from the following description.
To solve the foregoing problems, the inventors made extensive studies on the structure of triphenylamine derivative.
As a result, it was found that a compound obtained by replacing each one of the respective two phenyl groups connected to a nitrogen atom (N) at both terminals of the foregoing TPTE, i.e., two phenyl groups in total, by a naphthyl ring or higher aromatic condensed ring as shown in the following general formula (1): 
wherein R1, R2, R3, R4, R5 and R6 may be the same or different and each represents a hydrogen atom, alkyl group, halogenated alkyl group, aryl group, dialkylamino group or cyano group; and xcfx861 and xcfx862 may be the same or different and each represents an aromatic condensed ring which may have a substituent, has the following advantages:
(1) The compound has a glass transition temperature Tg of not lower than about 140xc2x0 C. and hence has a drastically improved heat resistance while maintaining the high hole-transporting capacity inherent to TPTE; and
(2) The compound hardly forms an exciplex assumingly because of its stereostructure having substituted large aromatic condensed rings at its terminals or because of the distribution of xcfx80 electron conjugated system.
The present invention has thus been worked out.
That is, the foregoing objects of the present invention have achieved by providing the following triphenylamine derivative and organic electroluminescence devices.
1) A triphenylamine derivative represented by the following general formula (1): 
wherein R1, R2, R3, R4, R5 and R6 may be the same or different and each represents a hydrogen atom, alkyl group, halogenated alkyl group, aryl group, dialkylamino group or cyano group; and xcfx861 and xcfx862 may be the same or different and each represents an aromatic condensed ring which may have a substituent.
2) An organic electroluminescence device comprising a cathode, an anode and an organic layer interposed between said electrodes, wherein said organic layer contains as a hole-transporting material a triphenylamine derivative represented by the following general formula (1): 
wherein R1, R2, R3, R4, R5 and R6 may be the same or different and each represents a hydrogen atom, alkyl group, halogenated alkyl group, aryl group, dialkylamino group or cyano group; and xcfx861 and xcfx862 may be the same or different and each represents an aromatic condensed ring which may have a substituent.
3) The organic electroluminescence device according to the above 2), wherein said organic layer comprises a single organic layer or a plurality of organic layers and at least one of said organic layers contains a triphenylamine derivative represented by general formula (1).
4) The organic electroluminescence device according to the above 3),
wherein said anode is an electrically-conductive transparent layer which comprises an electrically-conductive transparent material and which is formed on a substrate, and
wherein said layer containing the triphenylamine derivative is a hole-transporting layer provided on said electrically-conductive transparent layer directly or via a single hole-injecting layer.
5) The organic electroluminescence device according to the above 3), wherein said layer containing the triphenylamine derivative further contains at least one fluorescent dye.
6) The organic electroluminescence device according to the above 3), said organic layer comprises a layer containing as an electron-transporting material a 1,2,4-triazole derivative represented by the following general formula (2): 
wherein R7 and R8 may be the same or different and each represents a cyano group or diarylamino group; and n represents an integer of 1 or 2.
7) The organic electroluminescence device according to the above 2), wherein said triphenylamine derivative is represented by the following formula (1-3): 
Japanese Patent 2,851,185 discloses a device comprising a positive hole-transporting aromatic tertiary amine containing at least two tertiary amine components and at least two condensed aromatic rings connected to the nitrogen atom in the tertiary amine.
However, a further review of specific examples of the positive hole-transporting aromatic tertiary amine described in the 32nd column of the above cited patent shows that the tertiary amine thus proposed is nothing but one obtained by replacing at least two of phenyl groups or the like in a low molecular weight hole-transporting material having a unpolymerized base structure such as the foregoing TPD by condensed aromatic rings.
Further, the effect of the tertiary amine thus proposed is nothing but to improve the stability of the device as compared with the low molecular weight hole-transporting material. The above cited patent does not disclose or suggest improving the heat resistance of devices at high temperatures conditions together with improving the luminous efficiency thereof as attained in the present invention.
On the other hand, the present invention has been worked out on the basis of a new knowledge that by polymerizing the triphenylamine derivative to increase the resulting glass transition temperature Tg, the device has an unprecedentedly enhanced heat resistance at high temperatures and the formation of an exciplex accompanying the polymerization can be avoided on account of the condensed aromatic rings introduced into the molecule, to thereby enhance the luminous efficiency of the device. Accordingly, the present invention is not a mere combination of the foregoing polymerization technique developed by Tokito et al. and the condensed aromatic ring described in the above cited patent.